1. Field of the Invention
This invention relates to the field of wind turbines and, more specifically, to airfoils for use in the blades of such wind turbines.
2. Description of the Prior Art
Wind power has been used for ages to pump water, grind grain, and more recently to generate electricity. Such historic uses of wind power, however, have been primarily in applications where a single wind machine operated alone for the benefit of one or a small number of users. There is now more interest in developing wind powered generator systems in which electricity produced by a single wind powered generator or a group of wind powered generators can be supplied to utility power grids.
A conventional wind turbine for generating electric power typically includes two or more turbine blades or vanes connected to a central hub. The hub rotates about an axis and is connected to a shaft that drives an electric power generator. Wind turbines operate at either a constant rotational speed despite changes in wind velocity or at variable rotational speeds that are proportional to the wind velocity. Peak power at high wind speeds is usually controlled through stall regulation or through the use of variable pitch turbine blades.
The portion of the turbine blade closest to the hub is called the root of the blade, while the portion of the turbine blade farthest from the hub is called the tip of the blade. A cross-section of a turbine blade taken perpendicular to the imaginary line connecting the blade's root to the blade's tip is generally referred to as an airfoil. Theoretically, therefore, each turbine blade includes an infinite number of airfoils along the imaginary line. Typically, however, a blade's shape is defined in reference to a finite number of the airfoil shapes.
The geometric shape of an airfoil is usually expressed in tabular form in which the x, y coordinates of both the upper and lower surfaces of the airfoil at a given cross-section of the blade are measured with respect to the chord line, which is an imaginary line connecting the leading edge of the airfoil and the trailing edge of the airfoil. Both x and y coordinates are expressed as fractions of the chord length. Another important parameter of an airfoil is its thickness. The thickness of an airfoil refers to the maximum distance between the airfoil's upper surface and the airfoil's lower surface and is generally provided as a fraction of the airfoil's chord length. For example, a fourteen percent thick airfoil has a maximum thickness (i.e., a maximum distance between the airfoil's upper surface and the airfoil's lower surface) that is fourteen percent of the airfoil's chord length.
The chord length of an airfoil or cross-section of a turbine blade will typically become larger if the length of the blade increases and will typically become smaller if the length of the blade becomes smaller. Therefore, a table of coordinates for the geometry of the upper and lower surfaces of an airfoil remain valid for blades of different lengths, since the coordinates are dimensionless and are provided as percentages of the chord length of the airfoil.
Another important parameter for every airfoil or blade cross-section is its operating Reynolds number. The Reynolds number of an airfoil at a particular radial station is dimensionless and is defined by the following equation: ##EQU1## where R is the Reynolds number, c is the chord length of the airfoil, V is the flow velocity relative to the blade at the corresponding radial point on the blade, and .nu. is the kinematic viscosity of the air. Physically, the Reynolds number can be thought of as the ratio of the inertial force to the viscous force of air flow around a turbine blade. Viscous force is proportional to the shearing stress in the air flow divided by the rate of shearing strain, while inertial force is proportional to the product of the mass of the air flow multiplied by its acceleration.
Airfoil performance characteristics are expressed as a function of the airfoil's Reynolds number. As the length of a blade decreases, the blade's Reynolds number tends to decrease. For a particular airfoil along the blade span, a small Reynolds number indicates that viscous forces predominate while a large Reynolds number indicates that inertial forces predominate.
Conversion of wind power into electrical power is accomplished in most wind powered systems by connecting a wind-driven turbine to the shaft that drives an electric generator. In the past, conventional aircraft airfoil shapes have been used to design wind turbine blades. However, such aircraft airfoil shapes have created problems with wind turbines. For example, during clean blade conditions, when the blade has not been soiled with insects or airborne pollutants, aircraft airfoils can produce or generate excessive power in high winds, which can burn out electric generators. Another problem with using conventional aircraft airfoils for designing wind turbine blades occurs when the blades produce inadequate energy output due to the blades becoming soiled with insect accumulation and airborne pollutants. The soiling of the blades creates airfoil roughness, which adversely affects the airfoil maximum lift coefficient and the desired power output from the wind turbine. In aircraft, roughness is not a major concern, since aircraft typically fly in clean air at high altitudes and because of scheduled cleaning of the planes. Unfortunately, the blades of horizontal axis wind turbines (HAWT) typically become coated with insect accumulations and airborne contaminants. The collection of dirt and other materials on a wind turbine blade, otherwise called roughness, occurs predominantly at the leading edge of the blade. Roughness removal can be time consuming, difficult, and expensive. Furthermore, since the primary goal of a wind turbine is to covert the kinetic energy of the wind into electrical energy as inexpensively and efficiently as possible, any roughness of the rotors or blades reduces the operational efficiency of the wind turbine and diminishes its overall electric power or energy generating capabilities.
As a result of the problems described above, central to the use of airfoils in a wind turbine is the use of specially designed airfoils that govern the local airflow around the blade in a manner substantially different from conventional aircraft airfoils. While some advancements have been made in this area, as described in U.S. Pat. Nos. 5,417,548 and 5,562,420 issued to Tangler and Somers, both patents of which are specifically incorporated by reference into the present invention, further families of improved airfoils are still needed to shape and condition the local airflow around blades for more efficient operation and wind power conversion to electric power, and especially to be more insensitive to roughness effects.